The invention relates to an apparatus and method for improving the accuracy of Electrochemical Capacitance Voltage (ECV) profiling measurements by alerting the operator to the presence of surface films or gas bubbles during the etching process and by determining the true measurement area at the end of the measurement cycle and using the new value to recalculate the carrier concentration data. By making the area measurement integral to the ECV tool, every sample measurement can be corrected for the true measurement area, leading to improved accuracy and eliminating a large source of error. Further reduction in error is achieved by using the integral measurement system to monitor the sample surface during measurement for surface films and gas bubbles.
Semiconductor devices are made by sandwiching layers of material of different electrical and/or optical properties together. The layers are formed by epitaxial growth on or ion implantation or diffusion into a substrate wafer. Correct device operation necessitates close control of layer properties including carrier concentration and thickness. U.S. Pat. Nos. 4,028,207 & 4,168,212 describe the ECV profiling method, which is used for determining the carrier concentration as a function of depth into the layer and is therefore ideally suited to measuring these parameters.
ECV profiling makes use of the diode structure formed when a conducting liquid (electrolyte) is placed in contact with a semiconductor. The capacitance of the junction, in the reverse bias region, is determined by the magnitude of the applied bias and the carrier concentration vs. depth profile. By measuring this capacitance as a function of bias the carrier concentration depth profile can be determined. In this mode of operation the ECV profiler is similar to tools, which use metallisation or mercury in place of an electrolyte to form a diode structure. However, such tools are usually restricted to shallow depth profiles due to reverse bias breakdown of the semiconductor-metal junction. By using an electrolyte this limitation is overcome. The electrolyte is used to electrochemically etch into the sample, increasing the depth profiled without increasing the measurement bias. This makes ECV profiling a very powerful method for characterizing multi-layer structures.
Some of the factors which affect the accuracy and reproducibility of ECV profiling have been mentioned in “I. Mayes—Accuracy and Reproducibility of the Electrochemical Profiler, Mat Sci & Eng B80 (2001) 160-163”. This paper concludes that with ‘best practice’ the reproducibility of the method can be reduced to a standard deviation of around 2%. However, without careful control of the measurement a standard deviation of around 8% is more typical.
Usually a plastic ring, referred to as a sealing ring, defines the area of contact between the electrolyte and the semiconductor sample. Wear and distortion of this ring have a significant effect on the reproducibility of the contact area. In addition the contact pressure and wetting characteristics of the electrolyte have to be considered. All these factors influence the accuracy of the measurement.
Accurate measurements depend on knowing the precise area of contact between the test sample and the electrolyte and ensuring that it remains stable throughout the measurement process. As the carrier concentration, determined by this method, is inversely proportional to the area squared and the depth profiled is inversely proportional to area, large errors can occur. A 10% error in the contact area results in a 10% error in depth and more importantly a 20% error in the calculated carrier concentration. Although not all of the error can be corrected by measuring the final etch well area, routine measurement of this after each run significantly improves the repeatability of the measurement.
Gas bubbles and surface films can affect the measurement during profiling. Gas bubbles arise due to trapped air when the electrochemical cell is initially filled, or during electrolyte circulation or are a result of the electrochemical reactions used to etch the sample. Surface films arise from non-ideal electrochemical etching and are usually prevented by selecting a more appropriate electrolyte. Both gas bubbles and surface films lead to non-planer etching which changes the measurement area and can seriously affect measurement quality when multi-layer structures or structures with changing carrier concentration are being profiled. Gas bubbles can be removed by controlling the electrolyte flow and surface films can be prevented by careful control of the etching conditions. The etching bias, current and electrolyte flow rate are variables that can be controlled to alleviate these problems. Often the operator is not aware of the presence of gas bubbles or surface films and does not take action until their effect is seen in the electrical measurements, by which time the accuracy of the measurement is poor.
It is common practice to routinely check the sealing ring area by using an anodized n-type GaAs substrate, referred to as a ‘blue film slice’. The ‘blue film slice’ is profiled to form a shallow etch well, typically less than 0.5 microns deep. The slice is then removed from the tool and the area of the etched well is determined using a measuring microscope or some form of camera based metrology system. It is not the purpose of this invention to eliminate the need for this measurement as it provides important addition information on the quality of the seal and can be made without removing the sample from the tool using the apparatus described herein.
Electrochemically etching occurs when the sample is the anode (biased positively with respect to a counter electrode). For p-type material this is the forward bias condition and electrochemical etching readily takes place. For n-type material this is the reverse bias condition and electrochemical etching only occurs when the sample is illuminated with light of energy above the band-gap of the material.
Seepage under the edge of the ring means that the illuminated area is smaller than the area of contact between the sample and the electrolyte. This difference is important as the capacitance measurement is made over the whole of the sample area in contact with the liquid. The ‘blue film slice’ is used to measure the difference in these two areas. This excess area, Aexcess increases as the seal wears and is defined as:Aexcess=Awetted−Aillum 
Where Awetted is the contact area between the electrolyte and the sample (wetted area) and Aillum is the area illuminated by light, which for n-type material is the area profiled during the electrochemical etching process. The excess area is a small ring of material that lies under or close to the sealing ring edge. The capacitance associated with this ring of material gives rise to errors, especially for samples with a highly doped surface layer and lower doped interior, referred to as ‘Hi-Lo’ structures.
In addition to ‘blue film slice’ calibrations, some users measure the area of the etched well when the measurement is complete. The area of the sidewall is generally ignored, as the depth of the etched well is usually small in comparison with its diameter. Occasionally, the sample is also stylus profiled to check for roughness and to see that the etched well is planer. These measurements are made using other instruments such as a measuring microscope and a stylus profiler, which are not part of the ECV profiling tool. As a result these measurements are not always made and this leads to poor reproducibility. Further, without some means of viewing the sample surface during the measurement the conditions that lead to roughness and non-planer etching are difficult to avoid.